The issue of attention capture is of fundamental importance to understanding what we perceive and how we react when confronted with a salient, but previously unattended object or event. Two distinct, and almost entirely separate, literatures have examined this issue. One has measured implicit attention capture the effects of an unattended stimulus on performance of a primary task. Implicit capture studies determine whether attention capture has occurred by measuring whether or not the presence of a stimulus disrupts (or facilitates) performance on an attention-demanding primary task. Such studies address only the behavioral consequences of an irrelevant stimulus and do not systematically explore whether or not observers are aware of the capturing stimulus. The other literature measures explicit attention capture - the conscious awareness of a previously unanticipated stimulus. Both approaches are crucial to a complete understanding of capture in the laboratory and in the real world. However, in isolation, each literature has neglected critical aspects of capture. The proposed experiments use attention tasks developed in prior work on attention capture as well as several new paradigms in order to identify the factors that influence the explicit noticing of unexpected objects as well as subtle effects of irrelevant or unexpected events on behavior. In so doing, they address a number of open issues in both literatures. The studies focus on three main questions: (1) what role do expectations play in capture? (2) how does the nature of the critical object or event contribute to capture? and (3) how does engagement in a primary task contribute to capture? By varying the nature of the critical, unattended event, these studies will explore how capture is influenced by the behavioral relevance of the new object. Moreover, by varying the nature of the primary task and the degree of engagement required of subjects, these studies provide a more accurate picture of how attention capture operates in demanding and dynamic situations. Ideally, these studies will help to generalize results from simplified visual search tasks to more complex, real-world perception, thereby leading to a better understanding of the detection of important, but unexpected objects. By integrating approaches from both the implicit and explicit capture literatures, these studies may contribute to an improved understanding of both normal and damaged attentional processing (e.g., as in people with attention deficit disorder).